Conventionally, an aerobic biological treatment method, which is referred to as an activated sludge method, has been performed for treating waste water most generally. This method is carried out as shown in FIG. 4, wherein organic waste water, such as sewage, introduced from an organic waste water reservoir 100 into an aeration treatment device 102 is decomposed into an inorganic matter, such as carbon dioxide or water, under aerobic conditions in aeration tank 102 by "biological oxidation," which is oxidative decomposition by a microorganism. The waste water which has been treated in aeration tank 102 is separated into treated water 106 and sludge 108 by a precipitation device 104. Usually, a portion of sludge 108 is returned as a microorganism source to aeration tank 102, and the residual sludge (corresponding to a quantity of increased sludge in aeration tank 102) is discharged as an excess sludge 110.
In this case, a concentrated solution (i.e., sludge) containing an organic precipitated solid, which is solid-liquid separated by the precipitation device, is treated through steps of concentration, digestion, dehydration, composting, and incineration. Therefore, such a treatment is not preferred because it requires large amounts of time and expense.
For this reason, methods of decreasing an excess sludge as much as possible, such as an extended aeration method for increasing the residence time of a sludge, and a fixed bed oxidation method having microorganisms attached to a catalytic material surface of a media to hold a large quantity of the microorganisms in a reaction vessel (see "Sewage Service Project--Design Guide and Explanation" issued by Japan Sewage Works Association, edited by Sewage Works Division, Municipal Department of the Ministry of Construction, Vol. 2, 1994), have been proposed and commercialized. However, these methods require a large site area for increasing a residence time. In the extended aeration method, the sludge is distributed when a load is reduced, thereby interfering with solid-liquid separation. In the fixed bed oxidation method, fouling of the sludge is caused when the load is increased. Therefore, these methods are not desirable.
In order to solve these problems, a method of temporarily storing an excess sludge and decreasing the sludge by an anaerobic digestion method to reduce the quantity of sludge and to lessen the waste treatment burden has been proposed. (See, "Sewage Service Project--Design Guide and Explanation" issued by Japan Sewage Works Association, edited by Sewage Works Division, Municipal Department of the Ministry of Construction, Vol.2, 1994.) In this method, treatment time is long, for example 20 to 30 days, and the decrease in organic sludge is not sufficient, for example about 30 to 50%.
Japanese Provisional Patent Publication No. 6-206088 discloses a method of performing an aerobic treatment on organic waste water, then oxidizing a solid-liquid separated sludge by an ozone oxidation column to reduce excess sludge. According to this method, however, handling of ozone is complicated, and residual ozone should be treated. The decomposition rate of the excess sludge is not sufficient in the ozone oxidation column.